Oxygen production can be achieved by a variety of methods, e.g. distillation, absorption, membrane permeation, chemical reaction and diffusion. Almost all of these methods have been applied only to small scale oxygen production. For reasons of practicality and economic feasibility, air distillation is the only method currently employed to produce large quantities of oxygen having sufficient purity for commercial use such as in a coal gasification plant.
The apparatus used for the production of gaseous oxygen by air distillation generally is divided into five major zones. An air compression zone is utilized to compress air from atmospheric pressure to higher pressures needed for subsequent processing. An impurity removal zone is employed to remove water, carbon dioxide, hydrocarbons, and other impurities to thereby provide a highly pure air stream.
A third zone cools the air to its condensation temperature and recovers refrigeration through the expansion of gas by the use of heat exchangers. Through the use of a series of fractionation columns, the air is distilled in a fourth zone into the oxygen product and nitrogen waste. Finally, a fifth zone is employed to compress the oxygen product to the delivery pressure required by the end user.
One such system is described in Bernstein, U.S. Pat. No. 3,113,854 incorporated in its entirety herein by reference. Pressurized air is sent to a condenser to obtain a liquid/gas air product which is then fractionated in a high pressure column to produce a crude liquid oxygen product and relatively pure nitrogen gas. The crude liquid oxygen is sent to a low pressure fractionation column to obtain an oxygen product which is then warmed to ambient temperature and pressurized according to commercial requirements.
One aspect of the Bernstein process is to remove the liquid oxygen from the low pressure column and forward the same to the air condenser. The liquid oxygen is vaporized in the air condenser and a portion of the gaseous oxygen is used as reboil for the low pressure column.
Bernstein also provides for the controlled removal of a liquid material from the low pressure column for use in a reboiler to effect liquefaction of a high pressure gaseous nitrogen stream originating from the high pressure column. In addition, Bernstein provides refrigeration by expansion with work of a portion of the high pressure nitrogen gas using an expansion engine or turboexpander. In accordance with the Bernstein improvements of the conventional two fractionation zone separation system, the energy for air compression is significantly reduced.
However, the Bernstein process is able to obtain recoveries of pure oxygen greater than about 95 percent when the nitrogen flow to the turboexpander for refrigeration is less than 10 percent of the feed air, a value achievable only in the very largest plants. Considering the vast amount of pure oxygen needed for commercial applications, where flows to the turboexpander will be greater than about 10 percent, improving oxygen recovery remains a necessary and desirable goal in the industry. In addition, there is a need to operate an oxygen recovery system in which oxygen can be readily delivered over a range of pressures without utilizing an oxygen compressor; or where the feed gas to an oxygen compressor is at a pressure higher than that at which it could be delivered from low pressure column pressure. Such an improved method, and apparatus therefor is provided in accordance with the present invention.